1. Field of the Invention
The present invention generally relates to a communications network control method, and more particularly to a communications network control method suitable for controlling the physical layer and the data link layer in a LAN (Local Area Network) protocol included in the OSI standard.
2. Description of the Prior Art
In LANs, a plurality of stations access a single transmission path in order to communicate with other parties. Nowadays, various methods for accessing such a transmission path have been available. For example, a LAN using a CSMA/CD (Carrier Sense Multiple Access with Collision Detection) system has been proposed. Examples of such a LAN are an Ethernet and Star LAN prescribed in the IEEE 802.3. Such a LAN has an advantage in that the protocol used in the LAN is simple. In the CSMA/CD system, a station checks for carrier presence in the transmission path before transmitting. If a collision occurs, the station stops transmitting and becomes back-off. The CSMA/CD system has a small amount of delay and a high transmission efficiency at light traffic. However, collisions often occur at heavy traffic, and throughput is limited. Further, as traffic increases, the amount of delay increases, while throughput decreases. Hence, it is necessary to use a data rate much higher than traffic. This increases the cost of constructing the system.
A LAN using a token system is known. Examples of such a LAN are a LAN using a token bus prescribed in the IEEE 802.4 and a LAN using a token ring prescribed in the IEEE 802.5. In the token system, stations acquire the right to transmit data before transmitting. At heavy traffic, the token system has a smaller amount of delay and a lower limit on throughput than the CSMA/CD system. However, even at light traffic, a delay is not so small. In addition, the system needs a complex protocol and is thus expensive.
Japanese Patent Publication No. 58-40384 discloses a CSMA/CD system, in which a packet is destroyed if a collision occurs and communications continuously take place. Thus, the delay at heavy traffic does not increase greatly, and data can be efficiently transmitted. However, collisions are detected by comparing the address of an outgoing packet with the address of an incoming packet. Such a process for the above comparison increases the cost, and does not conform to the IEEE 802.3 standard, the disclosure of which is hereby incorporated by reference. Hence, a full-duplex function must be added to the standard. In general, it is required that communications systems be capable of being used for various purposes and compatible with the existing standards. Communications systems which do not meet the existing standards degrade the value of products in the market.
With the above in mind, Japanese Laid-Open Patent Publication No. 1-119224 has been proposed by an applicant who is the assignee of the present application. According to this proposal, node devices are connected so that a tree-like hierarchical structure is formed. When a node device receives inputs at low-order direction ports, the first received input port is given priority over the other input ports. Thereby, an input at a high-order direction port is given priority. In this manner, one communication is necessarily established. The detection of collisions is carried out by determining whether or not signals are present at the input and output ports. A network interface device meets the IEEE 802.3 standard, and is less expensive than that proposed in the Japanese Patent Publication No. 58-40384. The throughput does not decrease at heavy traffic, and is superior, in this regard, to the Ethernet using the CSMA/CD system.
FIG. 1 is a graph of a throughput versus delay characteristic, which is well known as a means for evaluating the performance of a bus-type LAN. The throughput versus delay characteristic shows the relationship between a transmitted communication quantity and the waiting time until the communication is completed. FIG. 1 shows the throughput versus delay characteristics of the Ethernet using the CSMA/CD system and the prior art proposed in the Japanese Laid-Open Patent Publication No. 1-119224.
In a case where there are stations A, B and X, if an input signal (packet) from the station A to the station B approximately concurs with an input signal (packet) from the station B and the station X, and the former input signal arrives at the highest-order node prior to the latter input signal, the station B is no longer capable of receiving packets from the station A. Hence, in a network having a small number of networks which often perform transmitting, the throughput characteristic is deteriorated.
FIG. 2 is a graph of a throughput characteristic of the system disclosed in the Japanese Laid-Open Patent Publication No. 1-119224 obtained at heavy traffic. The system has a problem at light traffic. Further, in the above-mentioned system, the station A cannot determine if packets transmitted by the station A have been received by the station B. This determination procedure depends on TCP. However, this degrades efficiency and responsivity.
Consequently, it is desired that in any case, the packet received by the highest-order node prior to the other packets be propagated through the network without being damaged, and thereby an ideal throughput characteristic be obtained at heavy traffic.